default templates should allow to reference existing templates
### Release notes
Improvement: reference existing templates for defining a default template
### Problem to solve
Current used version: GitLab Enterprise Edition 15.2.2-ee
A company defines several merge request templates to use in a shared repository.
This repository is configured as "template repository" on a group level (Group > Settings > General > Templates).
When a new merge request is created the user could choose of one of the stored templates.
But the user always HAS to choose one.
With default templates (Project > Settings > General > Merge requests > Default description template for merge requests | alternative default.md) you could define a default template to use. But you have to copy the content here.
### Intended users
* repository administrator
* developer who creates a merge request, issue (wherelse default templates are used)
### User experience goal
Developer could create a merge request without extra need to select a template.
Repository does not have to copy the default templates content to every project and therefore has DRY.
### Proposal
Provide a way to _reference_ an existing template.
### Further details
nothing extra
### Permissions and Security
unknown
### Documentation
### Availability & Testing
### Available Tier
all tiers which provide default templates
### Feature Usage Metrics
The new reference implies a new field. Setting this field could be measured.
### What does success look like, and how can we measure that?
The setting of the 'old' field with default template _content_ could also be measured, so you could see how many templates are migrated.
### What is the type of buyer?
### Is this a cross-stage feature?
### What is the competitive advantage or differentiation for this feature?
Use DRY in configuration.
### Links / references
issue